It is often useful in powering industrial and other machines to employ a transmission that allows for a smooth transmission through a series of effective transmission ratios. Such transmissions include what is known as a continuously variable transmission. This class of transmissions allows a somewhat continuously variable range of transmission ratios without excessive and distinctive “shifts” between fixed gears. One way of providing a CVT is employ a split torque system wherein one input to the transmission is the engine torque while a second input is from a device known as a variator. The variator is a hydraulic device that includes a hydraulic pump coupled to a hydraulic motor in such a way that the speed or torque output can be varied by varying a parameter of the pump, such as a swash plate setting.
In order to provide a full range of transmission ratios, the variator is typically required to provide torque in either direction as needed, and to be able to reverse the direction of rotation as quickly as possible during operation. Such an event is referred to as a torque reversal, and the point in time at which such an event occurs is referred to as a torque reversal point. Unfortunately, it is not possible with known technology to accurately and quickly reverse the variator torque. This failure to smoothly reverse the variator can lead to jerky and even unstable operation of the transmission.
One of the areas to smoothly reverse the torque lies internally within the variator, namely the hydraulic circuit or connection between the pump and motor. This system must accommodate hydraulic fluid at very high pressure, e.g., thousands of PSI, and must be able to allow the pressures to reverse on short notice. Because of the inefficiencies and imperfections in the system, a number of “make-up” valves are used to ensure that the system or circuit is always filled with hydraulic fluid from an external hydraulic source. The external hydraulic source of fluid may be required to deliver fluid to one part of the hydraulic circuit and then switch and almost instantaneously (e.g., within tens of milliseconds) deliver fluid to the second part of the hydraulic circuit. This means that at times, a first one of the make-up valves will be open and will supply make-up flow to the system, while at other times, the first valve will be closed and a second valve will instead supply make-up flow to the system. The shutting of one make-up valve and the opening of the other make-up valve has been found by the inventors to be a limiting factor in the torque reversal of the variator. In particular, these valves are traditionally one-way hydrostatic check valves, and as such, may experience a time lag before completely closing, thus disturbing or interrupting the torque or pressure reversal.
The hydraulic system or circuit also typically includes a flushing relief valve for returning hydraulic fluid from the hydraulic circuit to a supply tank. However, the response time of this valve is also a source of delay in controlling the variator or managing the torque, in that it may provide a pressure loss path before closing.
Efforts have been made to improve the torque reversal characteristics of the variator in such systems by improving the tolerances in the valves of interest and decreasing their damping and inertia. However, a suitable system has yet to be developed that sufficiently reduces the lag of the system valving, and as such, adequately improves the torque reversal operation of the variator.